Heres the question followed by the textbook answer from the back.

Im stuck on how it got the answer. Ive just done about 5 others in the same way, but this one doesn't seem to give me the right answer.

I got:

I did the correct method i think.

Heres the row of the pascal triangle i used:

1 6 15 20 15 6 1

So i basicly did:

... etc and simplified. I really don't understand how i got the answer wrong. Any suggestions?